Many new observations of seismic anisotropy in the upper and lower boundary layers
of the mantle are presented. These observations (from shear-wave splitting on SKS,
receiver corrected S, and receiver/source corrected ScS phases) constrain the mineralogy
and geodynamics in these regions, which are crucially important to mantle convection.
The main target of this thesis is the lower boundary layer: the lowermost mantle; where
the new (ScS) dataset greatly expands on the pre-existing coverage to reveal a global
dominance of SH fast anisotropy with strength ~1.4 %. Interesting deviations are
detected inside the Pacific large low shear velocity province where SV is fast; and in the
'slab graveyard' region beneath Eurasia where the angle of fast wave polarisation shows
coherent regional variations in dip suggesting the presence of large-scale (~1000 km)
structural features. A model of flow in the lowermost mantle is tested with the
hypothesis that the anisotropy is caused by the lattice preferred orientation of MgSi03
post-perovskite. It is demonstrated that the accuracy of ray theory is inadequate to test
general models of anisotropy in the lowermost mantle and that a full waveform finite
frequency method is required. Finite frequency waveform results do not match the
observations for three candidate post-perovskite plastic deformation models (dislocation
glide on (001), (010), and (100)/{11O}). Therefore either the flow model is wrong or
anisotropy in the lowermost mantle is not caused by dislocation glide deformation in
post-perovskite.
Anisotropy in the upper mantle causes on average 0.8 s of splitting beneath seismic
stations (determined from the SKS dataset). The fast wave tends to be polarised in line
with the direction of absolute plate motion in regions disturbed by orogeny in the last
540 Ma. This suggests that orogenies deform the mantle.
Beneath subduction zones, anisotropy causes an average 1.3 s of splitting in events
shallower than 300 km (from the S dataset). The fast direction tends to be aligned with
the strike of the slab (trench parallel splitting); notable exceptions are identified in South
America, the Izu-Bonin arc, a segment of the Sunda arc, and at the Hokkaido corner.
Interestingly, in each of these regions the trench is migrating forward in the direction
of the subducting plate's motion. With the exception of the Mariana arc, subduction
zones experiencing trench roll-back all display trench parallel shear wave splitting. This
suggests that trench parallel splitting is caused by mantle deformation associated with
trench roll-back. Events deeper than 300 km split by an average 0.9 s; this remains
true of events deeper than 520 km. The lack of depth dependence on splitting beyond
300 km hints that anisotropy is confined to the slab or is located in a region deeper than
~660 km.